High-impact poly(aromatic vinyl) resins such as high-impact polystyrene (HIPS) and ABS resins (acrylonitrile-butadiene-styrene resins) are polymer alloys generally obtained by blending or grafting an elastic polymer such as a conjugated diene copolymer with or on a poly(aromatic vinyl) resin to improve the impact resistance of the poly(aromatic vinyl) resin. Such a polymer alloy has a structure that the elastic polymer is dispersed in a matrix of the poly(aromatic vinyl) resin which is rigid and brittle, and is rigid and excellent in impact resistance.
High-impact poly(aromatic vinyl) resins obtained by using an aromatic vinyl-conjugated diene block copolymer as an elastic polymer have heretofore been known. The high-impact poly(aromatic vinyl) resins are generally produced by polymerizing an aromatic vinyl monomer (hereinafter referred to as "aromatic vinyl" merely) or a mixture of the aromatic vinyl and another monomer copolymerizable with the aromatic vinyl in the presence of an aromatic vinyl-conjugated diene block copolymer. As polymerization processes thereof, are used bulk polymerization, solution polymerization, bulk-suspension polymerization and the like.
However, such a high-impact poly(aromatic vinyl) resin obtained by using the aromatic vinyl-conjugated diene copolymer as an elastic polymer component has have poor susceptibility to coloring, which is considered to be attributable to insufficiency in evenly dispersing ability to colorants, and involved problems of haze development and high degree of opaqueness or whitening. In recent years, the high-impact poly(aromatic vinyl) resins have been used as housing materials for electric appliances such as televisions and air conditioners. In these uses, the high-impact poly(aromatic vinyl) resins are required to have excellent susceptibility to coloring and a low degree of opaqueness from the viewpoint of appearance.
A high-impact polystyrene resin obtained by using, as an elastic polymer component, a styrene-butadiene block copolymer, in which a molecular weight corresponding to a peak in a molecular weight distribution curve of a block styrene segment (i.e., styrene polymer block segment) as determined by gel permeation chromatography (GPC) is at least 30,000, and a proportion of block styrene segment portions having a molecular weight at most a third of the molecular weight corresponding to the peak is 25 to 50 mol % of the whole block styrene segment, has heretofore been proposed (Japanese Patent Application Laid-Open No. 74209/1989). However, this block copolymer is insufficient in improving effects on susceptibility to coloring and opacifying tendency.